The present invention relates to data processing systems, particularly those systems employing a named-data hierarchical memory system wherein data is generally named, stored and transferred between levels by blocks or pages of data words.
In such a hierarchical system a relatively small fast high cost per-bit memory system at one level is serviced by a relatively large slow low cost-per-bit memory system at a lower level. Data in use or likely to be used is transferred to the faster memory level one block or page at a time wherein a page contains, for example, eight contiguous data words.
In a named-data hierarchical memory system, a requestor submits the data page name to a translator to locate the data page in one level. If the translator does not locate the name the data is accessed from the next lower level and transferred to the higher level. The requested data name is inserted by the translator into an empty slot, if any, or by overwriting a currently stored data name according to any one of many replacement rules.
Frequently it is desirable to fetch or store only several of the data words to be received within a page. If the words are contiguous, the words may be fetched or stored by giving the page address which corresponds to the address of the first word to be accessed in the page and the number of subsequent contiguous words to be accessed. Thus a simple instruction could read in effect "fetch the first 4 words of the page named A". However, non-contiguous words are not readily fetched or stored. To fetch every other word of a page (for example) a sequence of instructions would have to be executed serially. The instructions would read in effect "fetch word 2 of page named A; fetch word 4 of page named A; etc." Thus a series of time consuming steps and the expenditure of considerable hardware and software resources is required to fetch a plurality of non-contiguous words in a page.
Further, prefetching of an entire data name and a data page from a lower level into a higher level is required before loading any words of a page into the higher level if the entire page is not already stored at that level.
Therefore, it is an object of the present invention to simplify and enhance the page transfer and replacement process in a named-data hierarchical memory system;
It is another object of the present invention to eliminate the need for page prefetching when a new but partial page is to be stored;
It is another object of the present invention to simplify and enhance the handling of non-contiguous words in a page oriented named-data hierarchical memory system.